I am fostering a very timid and fearful dog. When she is around the people she knows in a familiar environment?

she is a normal happy pup. Around strangers, she's generally very timid, BUT I took her to the pet store for the first time yesterday, and she was so scared she began shaking uncontrollably. She was brought in to the shelter as a stray. They say she is 3 yrs old, and she clearly has had at least one litter in the past. She is a great dog, and I'd like her to live a happy dog life without these irrational fears. I'm trying not to reinforce this behavior so I didn't comfort her when it happened; we just left because I didn't know what the right thing to do was. Any tips? Thanks.